1. Field of the Invention
The present invention relates to single-mode optical fibers used in optical transmission systems, and fabrication methods thereof.
2. Related Background Art
An optical fiber of a single mode type or the like is composed of a core region as a region in which light is transmitted, and a cladding region provided around the periphery thereof, and the optical fiber is constructed in such setting that the refractive index of the core region is slightly larger than that of the cladding region, thereby achieving optical transmission in the core region. Particularly, in the single-mode optical fibers the light is transmitted while optical power also spreads into the cladding region near the core region.
In the interfacial region being a boundary area between the core region and the cladding region, its refractive indices do not vary discontinuously but vary in a certain continuous index profile in which the refractive indices decrease from the core region toward the cladding region, in fact. Concerning such index change in the boundary, for example, Japanese Patent Application Laid-Open No. S49-17246 describes a layer with continuous index change provided in the boundary.
On the other hand, for example, Japanese Patent Applications Laid-Open No. S57-27934 and No. H03-8737 describe methods of fabricating a glass preform for optical fiber so as to reduce the thickness of the interfacial region (tail or tail spread).
With the interfacial region of the continuously changing index profile as described above, when the index change rate is small and when the thickness of the interfacial region is large, there arises a problem that the optical power transmitted, particularly, in the single-mode optical fibers is affected by the tail spread of index and the zero dispersion wavelength becomes longer than the working wavelength region. On the other hand, when the index change rate is large and when the thickness of the interfacial region is small, strain and heterogeneous portions remain inside the optical fiber after drawing because of a difference between viscosities of materials of the core region and cladding region, which will result in increasing transmission loss.
Namely, in the interfacial region there is a tradeoff relation between the limitation from the influence of the tail spread in a practical aspect and the limitation from occurrence of strain inside the optical fiber in a fabrication aspect, and it was difficult to overcome the both.
The present invention has been accomplished in view of the above problems and an object of the invention is to provide single-mode optical fibers having the interfacial region with adequate index change and achieving low loss phototransmission, and fabrication methods thereof.
For accomplishing the above object, a single-mode optical fiber according to the present invention is a single-mode optical fiber comprising a core region having a refractive index of n1 and a cladding region disposed around the periphery of the core region and having a refractive index of n2 to satisfy n2 less than n1, wherein a relative index difference in each part is determined with respect to the refractive index n2 of the cladding region, and a relative index difference of the core region is defined as xcex94n, wherein in an interfacial region with a continuously changing index profile near a boundary between the core region and the cladding region, for a domain in which a relative index difference varies from 0.8xc3x97xcex94n to 0.3xc3x97xcex94n, a change rate in relative index difference (0.5xc3x97xcex94n)/(d/r) normalized by a core radius r, where d is a thickness of the domain in a direction along a fiber diameter, and 0.5xc3x97xcex94n is a relative index difference change, is not less than 0.4% nor more than 4.0%.
A fabrication method of a single-mode optical fiber is a method of fabricating a single-mode optical fiber comprising a core region having a refractive index of n1 and a cladding region disposed around the periphery of the core region and having a refractive index of n2 to satisfy n2 less than n1, the method comprising: a forming step of forming the core region by a VAD method or an OVD method; and a selection step of selecting a transparent glass preform of the single-mode optical fiber including the core region, wherein in the selection step, a relative index difference in each part is determined with respect to the refractive index n2 of the cladding region, and a relative index difference of the core region is defined as xcex94n, and the transparent glass preform is selected so that in an interfacial region with a continuously changing index profile near a boundary between the core region and the cladding region, for a domain in which a relative index difference varies from 0.8xc3x97xcex94n to 0.3xc3x97xcex94n, a change rate in relative index difference (0.5xc3x97xcex94n)/(d/r) normalized by a core radius r, where d is a thickness of the domain in a direction along a fiber diameter, and 0.5xc3x97xcex94n is a relative index difference change, is not less than 0.4% nor more than 4.0%.
For determining a favorable condition for index change in the interfacial region in which the refractive indices decrease from the core region of the relative index difference xcex94n to the cladding region of the relative index difference of 0, a criterion for evaluation thereof is determined as the change rate in relative index difference defined for the region in which the relative index difference varies from 0.8xc3x97xcex94n to 0.3xc3x97xcex94n, and for values thereof the lower limit of the allowable range is set to 0.4% and the upper limit to 4.0%, thereby reducing the influence of the tail spread and the occurrence of strain in the optical fiber and thus realizing the single-mode optical fiber having favorable characteristics and capability of phototransmission with low transmission loss and the fabrication method thereof.
Namely, in order to realize the single-mode optical fiber capable of phototransmission with low loss, it is important to establish adequate evaluation and selection methods as to the index profile and thickness and as to the rate of index change based thereon for the interfacial region being the index changing region and to determine a favorable condition from the practical aspect and fabrication aspect. However, the aforementioned documents concerning the boundary and the tail spread fail to clearly describe the condition and others and include neither specific description nor investigation thereof.
In the present invention, the above region (80% to 30% of xcex94n) used as an evaluation criterion was set as a change range most suitable for the evaluation of change rate based on the evaluation that the inventors carried out using actual measurement results, as described hereinafter. When the transparent glass preform is selected based on the favorable condition of the index change determined as described above using the evaluation method, it becomes feasible to obtain the low loss single-mode optical fiber with certainty.
Since the relative index difference is normally expressed in percent (for example, about 0.3% in the case of the single-mode optical fibers), the change rate in relative index difference is also expressed similarly in percent. For the core radius r and the thickness d, measurement is carried out for the transparent glass preform for the optical fiber and they are expressed by the result of the measurement in units of mm or expressed as reduced values in units of xcexcm to the optical fiber after drawing.
The single-mode optical fibers with such index change are not limited to those consisting of a single core region and a cladding region. Namely, another single-mode optical fiber according to the present invention may be a single-mode optical fiber comprising an inner core region having a refractive index of n1, an outer core region disposed around the periphery of the inner core region and having a refractive index of n2 to satisfy n2 less than n1, and a cladding region disposed around the periphery of the outer core region, wherein a relative index difference in each part is determined with respect to the refractive index n2 of the outer core region, and a relative index difference of the inner core region is defined as xcex94n, wherein in an interfacial region with a continuously changing index profile near a boundary between the inner core region and the outer core region, for a domain in which a relative index difference varies from 0.8xc3x97xcex94n to 0.3xc3x97xcex94n, a change rate in relative index difference (0.5xc3x97xcex94n)/(d/r) normalized by an inner core radius r, where d is a thickness of the domain in a direction along a fiber diameter, and 0.5xc3x97xcex94n is a relative index difference change, is not less than 0.4% nor more than 4.0%.
Another fabrication method of a single-mode optical fiber may be a method of fabricating a single-mode optical fiber comprising an inner core region having a refractive index of n1, an outer core region disposed around the periphery of the inner core region and having a refractive index of n2 to satisfy n2 less than n1, and a cladding region disposed around the periphery of the outer core region, the method comprising: a forming step of forming the inner core region by a VAD method or an OVD method; and a selection step of selecting a transparent glass preform of the single-mode optical fiber including the inner core region, wherein in the selection step, a relative index difference in each part is determined with respect to the refractive index n2 of the outer core region, and a relative index difference of the inner core region is defined as xcex94n, and the transparent glass preform is selected so that in an interfacial region with a continuously changing index profile near a boundary between the inner core region and the outer core region, for a domain in which a relative index difference varies from 0.8xc3x97xcex94n to 0.3xc3x97xcex94n, a change rate in relative index difference (0.5xc3x97xcex94n)/(d/r) normalized by an inner core radius r, where d is a thickness of the domain in a direction along a fiber diameter, and 0.5xc3x97xcex94n is a relative index difference change, is not less than 0.4% nor more than 4.0%.
The single-mode optical fiber and the fabrication method thereof described above also provide the low loss single-mode optical fiber similarly.